Resistor



March 18, 1958 R LQOSJES 2,827,586

RESISTOR Filed June 29. 1954 INVENTOR ROBERT LOOS ES AGENT RESISTGR Robert Loosjes, Eindhoven, Netherlands, assignor, by

mesne assignments, to North American Philips Company, inn, New York, N. Y., a corporation of Delawere Application Eune 29, 1954, Serial No. 440,21?!

Qiaims priority, applicafiou Netherimds July 18, 1953 it Claims (1. 313235) This invention relates to resistors having a positive temperature coefficient of resistance.

it is known that metals have, in general, a positive temperature coefiicient of resistance. However, they have a low specific resistance which entails difficulties in proportioning if high resistance values are required.

Furthermore resistors are known that are made from semi-conducting, usually oxidic materials having a high resistivity. As a rule their temperature coefficient is negative, only a few compositions having a positive temperature coeficient in a limited range of temperature.

The present invention concerns resistors having a positive temperature coefiicient of resistance and a conduction mechanism which is fundamentally different from that of the aforesaid conventional resistors.

Researches leading to the present invention have revealed that in the case of porous oxide masses known per se which are serviceable for cathodes for use in discharge tubes in connection with their electron-emission with an increase in temperature, conduction at particular ternperatures at least considerably occurs through electrons present in the pores between the electron-emitting grains of the oxide mass. This conduction mechanism is sometimes termed pore-conduction. In order to ensure a satisfactory conduction of this type the mass should have a large pore volume and the pores should form continuous ducts through which electrons are allowed to travel between electrodes provided at the porous mass. Furthermore the diameter of the pores is of importance, since it determines the mean free path of the electrons and consequently the conductivity.

Again it had been found that the mean free path of electrons in a porous mass of the aforesaid type is reduced and consequently the resistance is increased by the presence, in the pores, of a neutral vapour whereof the molecules or atoms at a low pressure make the mean free path of slow electrons (a few tenths of an electron volt) smaller than the diameter of the pores.

If the porous member together with a solid or liquid material, giving off such a vapour at elevated temperatures, is introduced into an evacuated vessel, the vapour pressure will increase with an increase in temperature of the vessel, and consequently also the resistance of the porous member. In accordance with the vapourproducing material chosen the logarithm of the resistance of the porous member is directly proportional to in a given temperature zone.

The resistor according to the invention, which is based on what has been said above, comprises an evacuated vessel which contains a porous member made up of a material capable of electron-emission with an increase in temperature and provided between two electrodes passed through and insulated from the walls of the vessel, means for heating the porous member to the temperature re quired for electron-emission, and a material apt to be the vaporized and which at a pressure below 760 mm. yields a mean free path for slow electrons which is at least five times as small as the average diameter of the pores of the porous member. If desired, the vessel may also contain rare gases, for example argon.

The porous member preferably has a pore volume of 50% to 90%, the diameter of the pores being 0.1 to iUU r.

The material, of which the porous member is made up, may be a known electron-emitting material such as an alkali-and alkaline earth oxide, for example K 0, c5 0, Bat) and SrO alternatively in the form of mixtures or mixed crystals e. g. mixed crystals of BaO and SrO in a molar ratio of 1:1.

The porous member may be made from carbonates apt to be converted into the said oxides simply by heating. The desired porosity may, for example, be obtained by starting from a mixture of BaCO and SrCO with a particle size of from 1 to 10 This mixture may, for example, be pressed into the form of a tablet 0.3 mm. thick and 6 mm. in diameter whereof the ends are provided with nlcicel electrodes 100 thick, the assembly subsequently being heated to approximately 1000 centigrade in order to convert it into BaO-SrO mixed crystals. The obtained product has a pore volume of and an average pore diameter of approximately in.

In order that the invention may be readily carried into effect it will now be described with reference to the accompanying diagrammatic drawing.

in Pig. 1 a porous tablet 1 comprising hollow nic tel electrodes 2 is mounted by means of lead-through conductors 3 in a glass vessel 4, which constitutes a scaled enclosure. The hollow nickel electrodes contain tungsten heater coils 5 comprising supply conductors 6. This permits the porous member to be adjusted to the desired resistance value by adjusting the temperature. Furthermore, the vessel contains a supply of vaporizable material in the form of a drop of mercury 7 whereof the vapour is capable of influencing the mean free path of the conduction-electrons in the pores of the member 1. Alternatively, different materials, for example K, Na, Cs, may be used. Of course, said materials must not be capable of ionisation under the conditions prevailing in the vessel and must not react with the oxide material. As will be noted, the mercury supply 7 is separate from the porous member 1 and its heating means 5, so that its vapor pressure is more responsive to a temperature change of the vessel 4.

Finally the vessel, apart from the vapour pressure of the materials referred to and any present rare gases, is evacuated down to a pressure of at least 10* mm.

In Fig. 2 a slightly difierent form of the resistor is shown diagrammatically. In this case, a rod-shaped oxide member 1 is heated by means of a surrounding tungsten coil 5. The remaining parts bear the same reference numerals as in Fig. 1.

When using the resistors the porous member is first of all heated by the heater coils to a temperature of at least 800 K. for obtaining pore-conduction for a considerable part. At approximately lO00 K. conduction to as much as may occur by electron-displacement through the pores. The resistance of the porous member depends upon the temperature and is consequently controllable within wide limits. With the aforesaid oxide member the resistance is 76052 at a temperature of 1023 K. and i269 at a temperature of l148 K. if the temperature of the vessel is 301 K. At a constant temperature of the porous member the logarithm of the resistance is found to increase directly proportionally to 301 K, 458 K. and 623 K'., respectively. Conse quently an increase in resistance of about'200 times is obtained between 458K. and 623 K, which correspond to 185 centigrade and 350? centigrade, and of 2000 times between room temperatureand 623 K. or 350 centigrade,

Whatis claimed is:

1, A resistor havinga positive temperature coeilicient of resistance, comprising a'sealed enclosure, an electronproducing member in ;said enclosure having connected pores, electrodes coupled to said electron-producing member, means to cause said electron-producing member to produce electrons in said pores, and a supply of vaporizabie material in the enclosure but separate from the electron-producing member and the electron causing means, said supply of vapori'zable material including a portion in a state other than the vapor or gas state and providing'in said enclosure a vapor of said material whose pressure is a function of the temperature of an environment external, to said enclosure and separate from said electron-causing means, said vapor in the'pores of the electron-producing member reducing the mean free path of electrons in said pores.

2. A resistor having a positive temperature coeflicient of resistance, comprising a sealed enclosure, an electronemitting, porous mass in said enclosure and having connected pores, means within the enclosure to maintain said porous member at electron-emitting temperature thereby to produce free electrons in said pores, a supply of vaporizable' material in said enclosure and spaced from said temperature-maintaining means, said supply of vaporiza- -ble material including a portion in the liquid or solid state and providing in said enclosure a vapor of said material 7 whose pressure and density are dependent upon the temperature of the environment surrounding the enclosure, said pores having an average diameter at which the mean .free path of electrons therein is smaller than said average diameter by at least a factor of five due to the presence of a vapor of said material at a pressure below 760 mm.

of Hg, spaced electrodesconnectedto thcrporous memher, and means providing. external connections to said electrodes for connecting said resistor in an external cir- Cliit. 1 t

terial is mercury.

3'. The resistor of claim 1 wherein the vaporizable ma- 4. The resistor of claim 1 wherein the porous member is constituted of a material selected from the group consisting of alkali oxides and alkaline earth oxides.

5. The resistor of claim 1 wherein the porous member has a pore volume of to 6. The resistor or" claim lwherein the enclosure contains a rare gas.

7. A resistor having a positive coefiicient of resistance, comprising a sealed enclosure, a porous member in said enclosure constituted of a material selected from the group consisting of alkali oxides, alkaline earth oxides and mixtures thereof and adapted to produce electrons V at an elevated temperature, a pair of electrodes on opposite sides of said porous member, means for effecting external connections to said electrodes, means in the vi 7 cinity the porous member for heating the porous member to an elevated temperature, and a supply of vaporizahis material in the enclosure but spaced from the porous member producing in the enclosure a vapor which impedes the fiow of electrons in the porous member, thereby increasing the resistance of the porous member to electron flow between the electrodes, whereby the logarithm of the resistance of the resistor is directly proportional to in a given temperature zone, where T is temperature in degrees Kelvin.

8. The resistor of claim 2 wherein the pores of the porous member have a diameter of 0.1 1 to 9. The resistor of claim 7 wherein the porous member 7 consists of BaO-SrO mixed crystals.

10. The resistor of claim 7 wherein the vaporizable material is mercury.

References Cited in the file of this patent UNITED STATES PATENTS 1,634,420 Grondahl 'J'uly 5, 1927 1,839,899 'Slepian Ian. 5, 1932 2,162,478 Diamond Junel3, 1939 7 2,380,757 Horne l July 31, 1945 2,477,343 Postal' July 26, 1949 2,601,024 Jacobs et al June 17, 1952 2,688,648 Mcllvaine Sept. 7, 1954 2,747,127 Loosjes May 22,

Grondahl May 3, 1927 

